1. Field of the Invention
The present invention is directed to an integrated ammonia-urea process. More specifically, the present invention is directed to a process wherein the urea reaction of reacting carbon dioxide and ammonia is carried out in a reactor in the presence of raw ammonia synthesis gas. In the process of the present invention the urea reactor has a condensing section, a reaction section with more than one stage and a stripping section. The raw synthesis gas containing carbon dioxide is introduced into the stripping section of the urea reactor at a pressure within the range of about 2000 to 3500 psig.
2. Prior Art
The combination of an ammonia plant and a urea plant is common since the production of urea involves the reaction of two moles of ammonia for each mole of carbon dioxide. Commercially the combination of plants are relatively independent, i.e., the ammonia plant produces the ammonia and usually the carbon dioxide necessary for reaction in the urea plant. Various attempts have been made to integrate the two plants so that the two operations are integrated rather than merely combined.
U.S. Reissue 27,377 discloses an integration wherein the ammonia synthesis gas containing carbon dioxide is passed into a carbon dioxide absorber and the gases then passed on to the ammonia synthesis reactor. The carbon dioxide extracted in an ammonia and water solution is then reacted in the urea synthesis reactor.
The foregoing patent illustrates the basic integration which has been carried out in the prior art. The distinctions in the integrated processes have been primarily the definition of the specific stream utilized to extract the carbon dioxide from the reactant gases
U.S. Pat. No. 3,310,376 and U.S. Pat. No. 3,371,115 are both patents assigned to Chemical Construction Company showing various manners in which the carbon dioxide is extracted from a gas which is then reacted to form ammonia.
U.S. Pat. No. 3,372,189 assigned to Toyo Koatsu Industries likewise discloses extracting the carbon dioxide from an ammonia synthesis gas which is characterized by adjusting the mole ratio of ammonia to carbon dioxide in the resulting absorbate to between 2 to 3.6. Accordingly, the scrubbing column which removes the carbon dioxide is maintained under controlled conditions to properly select the composition of the aqueous absorbent solution and the temperature before feeding the aqueous absorbent solution to the scrubbing column.
U.S. Pat. Nos. 4,012,443; 4,138,434; 4,291,006; and 4,320,103 are all assigned to Snam Progetti. Each of these patents are directed to integrated urea-ammonia processes wherein the ammonia synthesis gas is first contacted with an aqueous solution usually containing ammonia to extract the carbon dioxide from the ammonia synthesis gas. U.S. Pat. No. 4,012,443 is specifically directed to utilizing two carbon dioxide absorbers. U.S. Pat. No. 4,138,434 is characterized as an improvement to the SNAM ammonia stripper integrated urea process wherein a gas stream obtained by reforming hydrocarbons is fed to an adiabatic ammonia stripper placed down stream of a carbamate decomposer. U.S. Pat. No. 4,291,006 is directed specifically to a process and apparatus for the absorption of the carbon dioxide in an apparatus which is divided into one section of a plate type absorber and the other section being a thin film type absorber. U.S. Pat. No. 4,320,103 is specifically directed to an improved method which consists in that portion of that gas stream comprising carbon dioxide, hydrogen and nitrogen being fed to the carbon dioxide absorption unit in a specified manner.
U.S. Pat. No. 4,235,816 also assigned to Snam Progetti is specifically directed to absorbing from the stream obtained by steam reforming of hydrocarbons the carbon dioxide in a very concentrated ammonia solution.
U.S. Pat. Nos. 3,607,939 and 3,647,872 both assigned to Stamicarbon are directed to procedures in handling the ammonia feed to the urea reactor. The ammonia synthesis gas referred to in these patents is the gas being removed from the ammonia reactor. It is understood that the process of producing ammonia involves a recycle of the ammonia synthesis gases since there is not a 100% conversion of the hydrogen and nitrogen to ammonia.
An article "The Urea Stripping Process--The Technical Manufacture of Urea, With Carbon Dioxide Used Both as Reactant and as Stripping Gas" by P. J. C. Kaasenbrood given at the Fourth European Symposium of the Chemical Reaction Engineering in Brussels in September 1968, is incorporated herein by reference in its entirety. This article discloses the technical preparation of urea as well as the stripping with carbon dioxide gas. The paper mentions the possibility of inerts but does not disclose any specific gases other than carbon dioxide as the gas for stripping the urea product.
U.S. Pat. No. 3,674,847 assigned to Stamicarbon discloses a process for urea production in combination with ammonia synthesis wherein a raw synthesis gas is utilized by introducing that gas into one of two strippers. The process is disclosed by reference to a urea plant with a capacity of approximately 1,800 metric tons of urea a day operated at a synthesis gas pressure of 350 atmospheres (approximately 5060 psig).
An article "Integrating Ammonia and Urea Production" by Douglas Keens published in I. Chem. E. Symposium, (series number 74) in 1982 is incorporated herein by reference in its entirety. This article discloses a number of proposals for the integration of ammonia and urea production. The author states: "It is curious that full integration of the two processes has not yet taken place on the industrial scale although the principles have been demonstrated as viable. One reason for this might be the unwillingness of compressor manufacturers to guarantee their machines for mixed gas streams containing carbon dioxide with water and/or ammonia present. Another might be the fact that the carbon content of the ammonia feedstock will not always be just right; a `lean` natural gas will give insufficient carbon dioxide, a petroleum feedstock will give too much. For flexibility, then, auxiliary carbon dioxide removal is needed, or means for handling surplus ammonia production; the increment in capital cost will work against some of the equipment savings made." This conclusion by the author was made after showing that all schemes discussed in the article had substantial reduction in capital cost and energy consumed.
It is still the state of the art that integration of the two processes has not taken place on the industrial scale. The present invention not only has the advantage of lower capital costs and energy savings, but addresses the practical reasons why the integration has not taken place heretofore.